| BACKGROUND:Pedicle screw fixation system for three columns fixation, and many other advantages has become the most widely used spinal fixation instruments. And it has certain therapeutic effect for spinal fractures, slippage, rebuilt after tumor resection, degenerative diseases, deformity correction and so on, becomes the gold standard of posterior spine stability. However, with the advent of aging society, Osteoporosis patients is increasing year by year. Osteoporosis vertebral compressive strength decreases obviously,this will seriously affect the nail - bone interface bonding strength, resulting in the screw loose and surgery failure. Therefore, how to improve the stability of pedicle screws under the condition of osteoporosis has become a great challenge to the spine surgery. At present,commonly using bone cement to strengthen pedicle screws to improve its stability in osteoporosis. However, in recent years, expansive pedicle screw has become one of the hot spot of current research, due to it can also improve the stability of the screw with no risk of leakage of bone cement. But the difference between the two aspects of the mechanical stability is not clear, the subject of a study around this problem.OBJECTIVE:Through the biomechanical study of expansive pedicle screw and fenestrated pedicle screw in the OP vertebral specimen to compare their maximum pullout strength, stiffness, energy absorption value, and clear the relationship between maximum pullout strength with bone mineral density. To investigate the stability and effectiveness of expansive pedicle screw and fenestrated pedicle screw, and provide theoretical foundation and biomechanical basis for clinical application.METHODS:Thirty vertebrae(T12-L5), excluding tumors, fractures and pathological changes, were obtained from 5 fresh-frozen human cadaveric spines with a mean of 70.4 years old. Bone mineral density (BMD) measurements of them confirmed the diagnostic criteria for osteoporosis. All vertebral specimens in the samples making, vertebral body biomechanics material selection, mechanical properties, measurement method, etc, were required to be same. The fenestrated pedicle screws were inserted into one pedicle of each vertebrae randomly, then bone cement 1.5ml per vertebrae was perfuse with the devices matched with the pedicle screw. It was the group FPS. The other pedicles were divided into the group of expansive pedicle screw(group EPS) and Ordinary pedicle screw(group CPS) randomly. Then The bone cement leakage situation of group A were observed, and the axial pullout strength were measured respectively.RESULTS:1. During the Pedicle screw and pin tract prefabricated process, it’s not occur pedicle fracture, rupture, perforation vertebral, in direct vision and X-ray examination found no leakage of bone cement.2. BMD Group FPS:0.686±0.025g/cm2, Group EPS:0.692±0.023g/cm2 and Group CPS: 0.680±0.026g/cm2. There were no significant differences (p>0.05) between any two groups.3. Maximum pullout strength There were significant differences (p<0.05) between any two groups.87.5% increase in group FPS than in group CPS,26.0% increase in group FPS than in group EPS,48.8% increase in group EPS than in group CPS.4. Stiffness Group FPS:222.700±31.887 N/mm, Group EPS:194.933±15.309N/mm and Group CPS:166.933±26.185N/mm. There were significant differences (p<0.05) between any two groups.5. Energy Absorption Group FPS:2.851±0.410J, Group EPS:2.043±0.378J and Group CPS:1.188±0.340J. There were significant differences (p<0.05) between any two groups.6. The maximum pull-out strength and BMD To a certain extent, the screw maximum output with the increase of BMD, pull the largest output and BMD have positive orrelation(p <0.05).CONCLUSION:Through biomechanical study in osteoporosis vertebral specimen found expansive pedicle screw and fenestrated pedicle screw all can significantly increase the stability of the screw, but fenestrated pedicle screws biomechanical stability is the best, and it can reduce the risk of leakage of bone cement. |
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